The present invention relates to protecting nickel-base alloys and components thereof from stress corrosion cracking when in contact with high temperature water. More particularly, the invention relates to protecting nickel-base alloy components of a boiling water reactor (BWR) from stress corrosion cracking when in contact with high temperature water. Even more particularly, the invention relates to protecting nickel-base alloy components of a boiling water reactor (BWR) from stress corrosion cracking when in contact with high temperature water by lowering the electrochemical corrosion potential of the nickel-base alloy components.
Nickel-base alloys, such as alloys 600, 690, 182, 82, X750, 718, and superalloys, have found applications in both boiling water nuclear reactors (hereinafter referred to as BWRs) and pressurized water nuclear reactors (hereinafter referred to as PWRs). These applications include use in many structural components found in nuclear reactors, such as, but not limited to, pipes, bolts, and weld material. Water for cooling the reactor core and extracting heat energy therefrom circulates within the BWR reactor pressure vessel, with about 15% of the water charged to steam. Inside the BWR reactor pressure vessel, the steam and circulating water typically have an operating pressure and temperature of about 7 MPa and 288xc2x0 C., respectively. For a PWR, the circulating water has an operating pressure of about 15 MPa and a temperature of about 320xc2x0 C. In the presence of water and/or steam under such high pressures and temperatures, components formed from nickel-base alloys are subject to intergranular stress corrosion cracking (hereinafter referred to as IGSCC), more commonly, or generically, referred to stress corrosion cracking (hereinafter referred to as SCC).
Stress corrosion cracking (SCC) of nuclear reactor components has long been a concern. As used herein, SCC refers to cracking propagated by the application of static or dynamic tensile stresses in combination with corrosion at a crack tip. The stresses encountered within BWR and PWR pressure vessels include those arising from the operating pressure for containment of the high temperature water in a liquid state, vibration, differences in thermal expansion, residual stress from welding, and fabrication-related sources of stress. Various materials and environmental conditions, such as water chemistry, welding, surface nature, crevice geometry, heat treatment, radiation, and other factors can also increase the susceptibility of reactor components to SCC.
Boiling water reactors use water as a means of cooling nuclear reactor cores and extracting heat energy produced by such reactor cores. Stress corrosion cracking is of particular concern in BWRs, as radiolytic decomposition of the high temperature water in the BWR core increases the concentrations of oxidizing agents, such as O2 and H2O2, in the high temperature water that circulates through the reactor. Consequently, the likelihood of extensive SCC in materials that are exposed to the high temperature reactor water is substantially increased. SCC can eventually lead to the failure of a nickel-base alloy structural component, such as a bolt. The premature failure of such components may lead to repeated or early shutdown of the reactor for part replacement or repair, thus reducing the amount of time the reactor is available for power generation.
The electrochemical corrosion potential (hereinafter referred to as ECP) affects the susceptibility of BWR components to SCC. The ECP is the mixed potential associated with the equilibrium of redox reactions occurring on a metal surface and the metal dissolution, and is dependent upon the amounts of oxidizing and reducing species present in the reactor water. In BWR reactor water, cathodic currents associated with the reduction of oxygen and hydrogen peroxide are balanced by anodic currents involving hydrogen oxidation and corrosion of metallic components.
Several approaches have been adopted to reduce SCC by lowering the ECP of the reactor water. In one such method, commonly referred to as hydrogen water chemistry (HWC), gaseous hydrogen is added to the BWR feedwater. Hydrogen addition reduces the oxidant concentrations, and thus reduces SCC susceptibility, by recombining with dissolved oxidants that are produced by the radiolysis of water in the reactor core. One disadvantage of HWC is that large amounts of hydrogen are needed to sufficiently lower the concentration of dissolved oxygen and to achieve a low corrosion potential. In addition, HWC can also increase radiation levels in the reactor steam by increasing the volatility of radioactive N16.
A second approach, known as noble metal technology (NMT), reduces the susceptibility of BWR components to stress corrosion cracking by lowering the corrosion potential more efficiently; i.e., by reducing the amount of hydrogen required to lower the electrochemical corrosion potential. The objective of NMT is to improve the catalytic properties for hydrogen/oxygen recombination on metal surfaces. Niederach (U.S. Pat. No. 5,130,080), Andresen and Niederach (U.S. Pat. Nos. 5,135,709 and 5,147,602), and Hettiarachchi (U.S. Pat. No. 5,818,893) have disclosed various NMT application methods, such as the thermal spraying of noble metal and noble metal alloy coatings on reactor components and noble metal chemical addition on metal reactor components. The NMT process lowers the corrosion potential to below xe2x88x92500mVSHE (standard hydrogen electrode) with a small amount of hydrogen addition. When combined with hydrogen addition in stoichiometric proportions or greater, complete recombination of oxygen and hydrogen peroxide on the catalytic surface of the noble metal is achieved and the corrosion potential is dramatically reduced.
Other methods, which do not require the addition of hydrogen to reduce the corrosion potential of reactor componentsxe2x80x94particularly of steel vessels and pipingxe2x80x94have been developed. Because electrically insulating films on metal surfaces reduce the corrosion potential, the ECP is also affected by the electrical conductivity of oxide films formed on metals in high temperature water. By lowering the electrochemical corrosion potential of metal components, the susceptibility of such materials to SCC can be significantly reduced. Andresen and Kim (U.S. Pat. No. 5,465,281) and Hettiarachchi (U.S. Pat. No. 5,774,516) teach a method of reducing the electrochemical corrosion potential of steel exposed to high temperature water with an insoluble and electrically non-conductive material, such as zirconia (ZrO2), alumina (Al2O3), or yttria-stabilized zirconia (YSZ) powders. However, air plasma spray coatings generally must be applied to the components either prior to installation or during a power outage. Moreover, it is difficult to achieve complete coverage with injection of insoluble chemical compounds into the reactor water.
More recently, Andresen and Kim (U.S. Pat. No. 6,024,805) have disclosed an in situ method of reducing the ECP and thus lowering the susceptibility of stainless steel that is exposed to high temperature water to stress corrosion cracking. The method includes the addition of a metal hydride to the high temperature water.
The prior art has focused on reducing in situ the corrosion potential of stainless steel pressure vessels and piping within BWRs. While insulating oxide coatings have been applied to nickel-base alloys, to date no attempt has been made to reduce in situ the susceptibility of nickel-base alloys to stress corrosion cracking by lowering the electrochemical potential of the alloy in the BWR without adding hydrogen to the reactor water. Therefore, what is needed is a method of lowering the susceptibility of nickel-base alloys that are exposed to high temperature water to SCC. What is also needed is a method of lowering the ECP of nickel-base alloys exposed to high temperature water, thereby mitigating stress corrosion cracking in such alloys. Finally, what is also needed is a nickel-based alloy having a reduced corrosion potential, and thus, a reduced susceptibility to stress corrosion cracking.
The present invention meets these needs and others by providing a method of reducing in situ the ECP of a nickel-base alloy that is in contact with high temperature water, such as in, but not limited to, the pressure vessel of a BWR, without adding hydrogen to the water. The present invention also provides an article formed from a nickel-base alloy having a reduced corrosion potential.
Accordingly, one aspect of the present invention is to provide a method for reducing in situ an electrochemical corrosion potential of a nickel-base alloy having a surface that is in contact with high temperature water, the electrochemical corrosion potential being proportional to the concentration of oxidizing species present in the high temperature water. The method comprises the steps of: adding a metal hydride to the high temperature water, the metal hydride being capable of dissociating in water; dissociating the metal hydride in the high temperature water to provide a metal and hydrogen ions; and reducing the concentration of the oxidizing species by reacting the hydrogen ions with the oxidizing species, thereby reducing in situ the electrochemical corrosion potential of the nickel-base alloy.
A second aspect of the present invention is to provide a method of reducing the in situ susceptibility of a nickel-base alloy component that is in contact with high temperature water in a boiling water nuclear reactor to stress corrosion cracking. The method comprises the steps of: adding a metal hydride to the high temperature water, the metal hydride being capable of dissociating in water; dissociating the metal hydride in the high temperature water to provide a metal and hydrogen ions; and incorporating the metal into an oxide layer disposed on a surface of the nickel-base alloy, the oxide layer being in contact with the high temperature water, wherein the electrical conductivity of the surface of the nickel-base alloy is reduced, and wherein the resulting decrease in the electrical conductivity reduces in situ the susceptibility of stress corrosion cracking of the nickel-base alloy component.
A third aspect of the present invention is to provide a method of reducing in situ susceptibility of stress corrosion cracking of a nickel-base alloy component that is in contact with high temperature water in a boiling water nuclear reactor, the susceptibility of stress corrosion cracking being proportional to the concentration of oxidizing species present in the high temperature water. The method comprises the steps of: adding a metal hydride to the high temperature water, the metal hydride being capable of dissociating in water; dissociating the metal hydride in the high temperature water to provide a metal and hydrogen ions; reducing the concentration of the oxidizing species, the oxidizing species being selected from the group consisting of O2 and H2O2, by reacting the hydrogen ions with the oxidizing species, thereby reducing in situ the electrochemical corrosion potential of the nickel-base alloy; and incorporating the metal into an oxide layer disposed on a surface of the nickel-base alloy, the oxide layer being in contact with the high temperature water, wherein the electrical conductivity of the surface of the nickel-base alloy is reduced, and wherein the resulting decrease in the electrical conductivity reduces in situ the susceptibility of stress corrosion cracking of the nickel-base alloy component.
Finally, a fourth aspect of the present invention is to provide a nickel-base alloy component having a reduced susceptibility to stress corrosion cracking when said nickel-base alloy component is in contact with high temperature water. The nickel-base alloy component comprises: a nickel-base alloy; a surface and a layer disposed thereon, the layer being formed from an oxide of a first metal and being in contact with the high temperature water; and at least a second metal incorporated in the layer, wherein the second metal is incorporated in situ into the layer by adding a hydride of the second metal to the high temperature water, dissociating the hydride in the high temperature water, and incorporating the second metal into said oxide of said first metal.
These and other aspects, advantages, and salient features of the invention will become apparent from the following detailed description, the accompanying figures, and the appended claims.